To solve these problems, the invention proposes producing MEMS sensors with resonator detection using “3D” planar technology.
The invention relates to a resonant MEMS sensor made by surface engineering, with excitation of the resonator in a plane, with multiple thicknesses. It is, for example, an accelerometer or a gyrometer. It is typically possible to have two different thicknesses on the same component, but also more than two thicknesses, for example three.
A first thick area has a first thickness, and is used for the seismic mass portion, for example an accelerometer or a gyrometer. A second thin area has a second thickness lower than the first. The ratio between these two thicknesses is, for example, between 2 and 10. The second area is used for detection and possibly for the production of hinges (or a torsion axis in the case of a gyroscope).
According to the invention, the design of the resonant sensors made by surface engineering is modified by decorrelating the seismic mass part from the detection part.
The first area preferably has a surface S smaller than 0.1 mm2 for an accelerometer and smaller than 5 mm2 for a gyrometer.
The first and the second areas can be formed in the superficial semiconductor layer of a SOI substrate.
A sensor according to the invention can also comprise a third area forming a hinge (or a torsion axis in the case of a gyrometer), said third area having a thickness between that of the first area and that of the second area, or equal to that of the first area, or equal to that of the second area.
The invention makes it possible to separately optimise various parts of the sensor:                the resonator(s),        the hinge(s) or the torsion axis,        the seismic mass.        
It is also compatible with a very inexpensive “In-IC” approach.
Mechanical stops can be provided to limit the movement of the seismic mass.
Digital detection means can also be provided, for example comprising means for digital filtering and/or digital processing of the signal.
The invention also relates to a method for producing a device according to the invention, implementing etching steps that are differentiated, in depth, perpendicular to a plane of the seismic mass, of the first and/or second and/or third areas.
The invention relates in particular to a method for producing a surface-type MEMS resonant sensor, comprising a resonator with excitation in the plane, which method comprises:                the formation of a thick area, having a first thickness (E1), forming a seismic mass,        the formation of a thin area, having a second thickness (E2), lower than the first, for detection.        
The thick area and the thin area can be formed by etching a layer of semiconductor material in three dimensions.
They can be formed by etching a superficial layer of semiconductor material of an SOI substrate, in a direction (z) perpendicular to a main plane of said SOI substrate.
Such a method can also comprise the formation of a third area, called the hinge or torsion axis area, with a thickness between that of the first area and that of the second area, or equal to one of the thicknesses of said two areas.
The thin area is advantageously produced in a superficial semiconductor layer, with the thick area being produced in an epitaxial semiconductor layer on said superficial layer and in said superficial layer.